Modern hearing aids are often provided with some sort of noise reduction scheme based on directionality or signal processing blocking out noise signals. Also in other assistive listening devices such as hand held microphone systems noise reduction is often utilized.
With regard to the invention it is important to distinguish between noise reduction algorithms that apply to a single sensor signal and noise reduction systems that employ two or more sensor signals.
The former category of noise reduction algorithms exploits the fact that a speech signal has certain distinct characteristics that are different from the characteristics of most noise signals. Hence, if the noise is speech-like (other voices, for example) the noise reduction algorithm will have no effect. Also they are characterized by dividing the input signal into a number of frequency bands. In each frequency band, an estimate of the modulation index (or something similar) is used to predict whether there is useful speech information available in that band, or whether the band is dominated by noise. In bands dominated by noise the gain is reduced. It is clear that in each frequency band it is impossible to improve neither the local Signal to Noise Ratio (SNR) nor the local Speech Intelligibility (SI). Thus, the algorithm can only improve the global SNR/SI by attenuating bands with so much noise that they mask out the useful speech information in other bands. Accordingly, such noise reduction algorithms that have been implemented in hearing aids have not been able to provide systematic improvements of SI, but only improved listening comfort (Boymans, M., W. A. Dreschler, P. Schoneveld & H. Verschuure, 1999, “Clinical evaluation of a fully-digital in-the-ear hearing instrument”, Audiology 38(2), p. 99-108. Boymans, M. & W. A. Dreschler, 2000, “Field trials using a digital hearing aid with active noise reduction and dual-microphone directionality”, Audiology 39(5), p. 260-268. Gabriel. B., 2001, “Nutzen moderner Hörgeräte-Features für Hörgeräte-Träger am Beispiel eines speziellen Hörgeräte-Typs”, Z. Audiol. 40(1), p. 16-31. Valente, M., D. Fabry, L. Potts & R. Sandlin, 1998, “Comparing the performance of the Widex Senso digital hearing aid with analog hearing aids”, Journ. Am. Acad. Audiol. 9(5), p. 342-360. Walden, B E., R K. Surr, M T. Cord, B. Edwards & L. Olson, 2000, “Comparison of benefits provided by different hearing aid technologies”, Journ. Am. Acad. Audiol. 11, p. 540-560.).
In contrast, noise reduction systems that employ two or more sensor signals exploit the spatial differences between the target and noise sources. By combining these input signals it is possible to remove signal contributions impinging from non-target directions, which means that both SNR and SI can be improved both locally and globally in the frequency range of operation (Killion, M., R. Schulein, L. Christensen, D. Fabry, L. Revitt, P. Niquette & K. Ching, 1998, “Real-world performance of an ITE directional microphone”, The Hearing Journal, 51(4). Soede, W., F. A. Bilsen & A. J. Berkhout, 1993, “Assessment of a directional microphone array for hearing-impaired listeners”, J. Acoust. Soc. Am. 94(2), p. 799-808.). The present invention regards only the latter category of noise reduction systems.
The signal processing in noise reduction systems which are based on directionality can be either fixed-weight or adaptive. In a fixed-weight system, the directional pattern is designed once and for all, based on some assumptions on the nature of the typical noise sound field, e.g. that the noise sound field is diffuse. In an adaptive system, the directional pattern is adjusted online according to some optimization scheme. Either way, such noise reduction systems have so far been designed to function over a broad frequency range, and in the signal processing unit of the hearing aid the output signal is subjected to a certain amount of amplification, which is determined according to the hearing loss of the individual carrying the hearing aid.
An example of a traditional way of realizing an adaptive beamforming is given in U.S. Pat. No. 4,956,867 and in WO 00/30404 where equal priority is given to all frequencies.
While these two examples consider broadside arrays, an adaptive endfire array is disclosed in U.S. Pat. No. 6,154,552.
It has not hitherto been suggested to tailor the noise reduction to the hearing loss of the individual and no methods for doing so have been proposed.
In a study by Saunders G H and Kates J M published in 1997 in an article in “Journal of the Acoustical Society of America” 102:3; 1827-1837 the performance of directional systems used by hearing impaired subjects are compared. In the study Saunders and Kates ran a series of speech reception threshold and speech intelligibility rating experiments with eighteen hearing impaired subjects with symmetrical sloping hearing loss. They processed separately recorded microphone signals from five microphones in an equally spaced 11-cm endfire configuration. The signals were recorded in an office room and a (more reverberant) conference room and processed off-line in two directional array systems (delay-and-sum and superdirective). The two arrays were compared to a cardioid and an omnidirectional microphone.
FIG. 1 shows the result of speech intelligibility tests for hearing impaired subjects in eight situations wherein two directional alaorithms, i.e., delay-and-sum (DAS) and superdirective (SUP). were tested against a cardioid (CAR) and an omni-directional microphone (OMN). The figure demonstrates that the superdirective system (SUP) performed best in both listening situations (office and conference room). However, contrary to the authors' expectations, the delay-and-sum (DAS) performed worse than a single cardloid microphone (CAR), although the directivity index of the cardioid microphone when weighted with the articulation index (AI-DI) was inferior.
Saunders and Kates pointed out that at low frequencies, the directionality of a cardioid microphone is better than the directionality of the delay-and-sum array. They speculated that their surprising result could be explained by the speech power, which is concentrated at low frequencies. This is however inconsistent with the articulation index importance function, which shows dominance at higher frequencies as seen in FIG. 2.
On the basis of the results from the above study it is not clear how a noise reduction should be tailored to give the most benefit for a particular kind of hearing loss.
An object of the invention is to provide a method of tailoring noise reduction to the individual hearing impaired person, such that maximum benefit of the noise reduction is obtained for the hearing impaired.
A further object of the invention is to provide a hearing aid or a listening device suited to perform a noise reduction tailored to the hearing loss of the individual using the device.